Hypoxia is a condition in which the body or a region of the body is deprived of an adequate oxygen supply needed to sustain cognitive brain function and prevent tissue and organ damage. Generalized hypoxia can occur in healthy people during flight, due to a decrease in the partial pressure of oxygen in inspired air as altitude increases beyond the limits of human physiological compatibility. Hypoxia, or altitude sickness, is the number two leading cause of aircraft mishaps, and can lead to potentially fatal complications, such as high altitude pulmonary edema (HAPE), high altitude cerebral edema (HACE), and ultimately hypoxic loss of consciousness (HLOC).
For years, cabin pressurization and onboard oxygen systems have allowed for normal in-flight high altitude aviation activities by aircrew, within the aircraft. However, flight safety with regard to hypoxia, is still limited by the failure of the onboard flight oxygen equipment, and aircraft structural failures. For example, most commercial aircraft maintain a cabin pressure of 8,000 ft, and all unpressurized aircraft operate below a 10,000 ft. ceiling, which are ideal for preventing hypoxia from occurring. Combat aircraft has a higher altitude operational ceiling, and their onboard oxygen systems are used invariably, until a safe breathable altitude is reached. The threat of hypoxia is ever present with an accidental loss of cabin pressurization, or when flying unpressurized aircraft beyond the maximum altitude in which the ambient air is not suitable for unaided breathing. On Aug. 14, 2003, 6 crew and 115 passengers perished in the crash of Greece Helios Airways Boeing 737-300 due to loss of cabin pressurization. This hypoxia related accident along with many others, is why hypoxia recognition and recovery, remains a major interest in both military and civil aviation training.
Aviation related hypoxia is known as hypobaric hypoxia, which is caused by breathing air at altitudes above 10,000 feet. When altitude increases, the partial pressure of oxygen, in the inspired air, is progressively reduced. This is compared to a normal barometric pressure of 160 mm Hg and a 20.9% oxygen concentration while breathing air at sea level. The typical causes of accidental hypoxia in flight include: ascent to altitude without a supplemental oxygen supply, failure of personal or aircraft oxygen breathing equipment, or decompression (loss) of aircraft cabin pressure.
The signs and symptoms of hypoxia become apparent as the degree of hypoxia increases. This can include: shortness of breath, air hunger, excessive yawning, tiredness, fatigue, euphoria, physical impairment, mental impairment, altered phisio-sensory mechanisms, or any combination of these, which can ultimately lead to a complete loss of consciousness (HLOC). An individual's hypoxia symptoms are affected by many different physiological factors, which differ for from person or person. Among them, the factors include varied flight dynamics, such as, altitude, rate of ascent, duration at an altitude, ambient temperature, the physical activity of an individual, the individual's own unique susceptibility, his/her health and physical fitness. There is no noticeable symptom of discomfort or pain associated with the onset of hypoxia. It is therefore vital, that each flight crew member is trained to recognize his/her “individual” hypoxia symptoms, as the onset symptoms of hypoxia can be insidious subtle and can begin without any conscience warning.
The key to dealing with the altitude sickness is taking advantage of the body's ability to gradually acclimatize slowly through a transition of progressively higher altitudes. The body adjusts to altitude by increasing respiratory volume, increasing pulmonary artery pressure, cardiac output, the number of red blood cells, oxygen carrying capability of red blood cells, and by even changing body tissues to promote normal function at lower oxygen levels.
Low pressure chambers are typically used in the United States for aviation hypoxia training. Nearly 10,000 students receive hypobaric training in the U.S. Navy annually. The training consists of exposure to hypobaric environments at or above altitudes of 20,000 feet. The incidence of decompression illness resulting from hypobaric chamber training has been reported by a number of military training organizations. A review of 10 of these reports shows a range of incidence in various populations from 0.3 to 2.9 cases per 1000 exposures, with a mean incidence of 1 case per 1000 exposures (or 0.1%). The Navy has average 4 cases of Decompression Sickness (DCS) annually in its hypobaric chambers with an associated cost of several thousand dollars per treatment, and the possibility of long term medical complications for the patient. These chambers are expensive to construct and operate, and only a limited number of these chambers are available. Despite their relatively large size, the chambers are still relatively small to allow incorporation of mission simulators into the hypoxic environment. Some investigators believe that if hypoxia training and flight training could be combined, the realism of the training scenario would be greatly improved, and the overall training benefit would be significantly increased for aircrew.
The U.S. Navy has had outstanding success in using mixed gas (normobaric hypoxia) mask on training devices for hypoxia recognition/recovery training for tactical jet aviators, while using the Reduced Oxygen Breathing Device (herein “ROBD” as described in U.S. Pat. No. 6,871,645 and “ROBD2” as described in US Pub. 20050247311). ROBD/ROBD2 training devices uses an aviator's mask to deliver a reduced oxygen content mixed gas, to the individual aviator. This gas mix is adjusted to increase or decrease the oxygen concentration, for any altitude for which the ROBD/ROBD2 is programmed to attain. Although ROBD units can reduce the danger of decompression illness, caused by hypobaric chambers, they are less suitable for training of multi-crew pressurized aircraft aviator aircrew. The hypoxia training for pressurized aircraft crew training would most likely involve a “mask off” hypoxia training scenario. It is of paramount importance that aircrew communication and coordination is practiced while training multi-crew pressurized aircraft students on hypoxia recognition and recovery. Therefore, there is a need for a realistic, mask-off, sea-level (normobaric) hypoxia training system. This type of hypoxia training environment alleviates any aircrew student from ever having decompression sickness occur, as there is no barometric pressure change within a hypoxia enclosure/room, as opposed to the barometric pressure changes experienced while aircrew train in a “hypobaric” altitude chamber.
Several companies have developed normobaric (no barometric pressure change) reduced oxygen concentration training environment, in a relatively large sealed space. These reduced oxygen rooms are capable of maintaining reduced oxygen environments, which emulate altitudes in excess of 30K feet. For example, Colorado Altitude Training LLC (Louisville, Colo.) has built several product lines for sport, military and aviation purposes, including a hypoxic sleeping system, a hypoxic exercising system, a free-standing enclosure system, an environmental chamber conversion system, a hyperoxic (oxygen-rich) system, and an aviation systems simulating up to 30,000 feet. An example of a CAT system and a method for passive hypoxic training is described in U.S. Pat. No. 6,827,760 to Kutt. The 760's system comprises an oxygen concentrator, sensors for oxygen, temperature, CO2 and ambient pressure, and a CO2 scrubber, which eliminates CO2 to keep the air fresh and clean within the chamber. Also included in the 760's system is a ventilation fan, a vent, a gate, and a blower, which brings in fresh air when oxygen levels fall below desired levels, or when carbon dioxide levels rise above desired levels, and if either oxygen or CO2 are outside of their safe range. A controller is used to regulate the oxygen concentrator, the CO2 scrubber, and the ventilation fan so the percentage of oxygen in the room can be altered by removing carbon dioxide, and bringing in fresh air. The controller also monitors oxygen and carbon dioxide levels.
Colorado Altitude Training (CAT) LLC., aviation hypoxic training chamber is based on a double tent design, which may be adapted from any sufficiently sealed space. A pressure transducer is used to determine the natural elevation. The controllers are self-calibrating, can provide up to 42 days of data logging, and have remote display capability. In current sports and athletic hypoxia training, using the CAT room, hypoxia recovery is accomplished using a medical grade oxygen mask that supplies 100% O2. Although, the CAT hypoxia recovery system employs the emergency oxygen masks found on commercial airplanes, and in other commercially available hypobaric attitude chambers, because the user is wearing an air mask throughout the training, it fails simulate realistic conditions of hypoxia event for the aircrew of a pressurized aircraft and thus not suited for mask-off hypoxia recovery training. Furthermore, the operations of these chambers are both expensive and labor intensive. The recovery system often requires large volumes of onsite oxygen storage, as 100% oxygen is supplied to multiple aircrew trainees and instructor, during the hypoxia recovery training. The oxygen storage requirement is particularly unfeasible due to strict restriction under fire code regulations regarding storing and supplying large quantities of 100% oxygen. A typical mask-off hypoxia training chamber needs an OSHA approved gas storage room to be build. Bank of multiple “T-bottles” hospital grade oxygen need to be connected to the chamber via manifold and plumbing. The system is also expensive to operate. Four bottles of oxygen only last about 10-15 lab sessions, and need to be automatically switches to the other oxygen source when line pressure reaches a threshold.
The current invention aims to alleviate many of the problems associated with the current hypoxia recovery systems. A mask-off hypoxia system for a pressurized aircraft of this invention, would provide realistic simulations of different hypoxia events, and allow multi-crew hypoxia recovery drills, which may include drills on hypoxia recognition, crew-communication and hypoxia recovery.